1. Field of the Invention
This invention relates to a cutting tool assembly and more particularly to a replaceable cutting tool assembly that is supported by a base member affixed to a driving mechanism in which relative movement between the cutting tool assembly and the base member is prevented to prevent wear of the base member and the need to replace the base member on the driving mechanism.
2. Description of the Prior Art
It is a well known practice in mining operations to mount a plurality of cutting elements on the periphery of a driving mechanism, such as a cylindrical drum, an endless chain, or the like that upon rotation drives the cutting elements into contact with a mine face to dislodge the solid material from the mine face in carrying out the mining operation. Each of the cutting elements includes a bit fabricated of a hard material such as tungsten carbide or other carbide materials. The bit is suitably supported in a cutting position to extend outwardly at a preselected angle from the driving mechanism so that upon rotation of the driving mechanism the bits strike the mine face at an angle to dislodge the mine material from the mine face.
The cutter bits are generally conical in configuration. In one embodiment the the cutter bit includes a shank which is adapted to be detachably received in a socket member of a bit holder that is secured to the periphery of the driving mechanism, such as the surface of a mining machine cutter drum. In another embodiment a conical bit is inserted in a holder that includes a shank portion releasably positioned in the socket of a base member or lug that is secured to the driving mechanism, as for example by welding the lug to the driving mechanism.
The cutter bits are subject to wear under the stresses generated by the cutting operation. Consequently after periods of use the cutter bits become dull or broken and inoperable to efficiently dislodge solid material from the mine face. It is the customary practice to replace dull or broken cutter bits with replacement bits. To facilitate rapid replacement of a worn cutter bit in a base member or lug a variety of "knock-out" bits have been developed to reduce the downtime of the mining machine for bit replacement.
The stresses generated by the cutting operation on the cutter bits not only accelerate wear of the cutter bits but also generate wear of the base member that is welded to the driving machine. When worn base members are not replaced the efficiency of the mining operation is also impaired. Because the base members or lugs are secured by welding to the driving mechanism, their replacement is difficult, resulting in considerable downtime for the mining machine. One approach to reducing wear of the base members and the corresponding downtime in their replacement has been to rotatably support the cutter bit in the base member. This arrangement is illustrated in U.S. Pat. No. 3,397,012.
It is also known to rotatably position a cutter bit within a bore of a bit holder, and rotatably position the bit holder within the bore of a support block that is welded to the surface of a drum, as illustrated in U.S. Pat. Re. No. 29,900. With this arrangement the bit holder is axially and rotatably movable within the bore of the support block. Consequently, the support block is subject to wear requiring its replacement which, as above discussed, is difficult and time consuming.
U.S. Pat. No. 3,690,728 discloses a spring clip for securing the shank of a cutter bit, either rotatably or nonrotatably, in the shank receiving perforation of a lug-type tool holder assembly. Nevertheless, the shank receiving perforation is still subject to wear. When wear of the tool holder begins to diminish the efficiency of the mining machine, then the tool holder must be replaced.
It has been the approach in the past to rotatably position the bit holder in the bore of the base member to provide uniform wear of the cutter bit. This approach is disclosed in U.S. Pat. Nos. 3,342,531; 3,342,532; 3,992,061; and Re. 29,900. For the bit holder to be rotatable it is necessary that the bit holder be fully seated in the bore of the base member. If the seating of the bit holder in the bore is improper or a slight misalignment exists between the complementary bearing surfaces of the bit holder and the base member, then the bit holder and the base member will be become worn under the stresses of the cutting operation.
While replacement of a worn bit holder consumes a minimum of time, replacement of a worn base member is very time consuming. The worn base member, which is welded to the driving mechanism, must first be removed. The replacement base member must be positioned in the proper location on the driving mechanism to coordinate the location of the cutter bit with the other cutter bits on the driving mechanism. Then the replacement base member must be welded to the surface of the driving mechanism. Consequently, the mining machine is removed from operation for a considerable period of time during the replacement operation.
It is also known to reduce the stresses exerted on the base member during the mining operation by forming complementary bearing surfaces between either the cutter bit or the bit holder and the base member. Cutter bit assemblies having complementary bearing surfaces are disclosed in U.S. Pat. Nos. 3,342,531; 3,342,532; 3,992,061; and Re. 29,900. The complementary bearing surfaces are generally formed by a tapered body portion of the cutter bit or bit holder engaging a frusto-conical seat in the the base member. Also it is known to provide complementary bearing surfaces between the cutter bit and the bit holder to ensure that the cutter bit is fully seated in the bit holder but rotatable in the bit holder.
For the arrangement in which the tapered body of the bit holder engages the frusto-conical seat of the base member, axial movement of the bit holder in the base member is permitted so that the bit holder will rotate in the base member. However, if the complementary bearing surfaces are not fully engaged and are retained in a misaligned position, then the complementary bearing surfaces will wear relatively rapidly. As a result the bit holder and the base member will require replacement.
There is need in mining operations for a cutter bit assembly including a cutter bit and bit holder that are replaceable in a base member affixed to the driving mechanism of the mining machine in a manner that substantially reduces the wear of the base member and the frequency of replacement of the base member in order to reduce the downtime for the mining machine. While it has been suggested to provide complementary bearing surfaces between a rotatable cutter bit or bit holder and the base member, elaborate retention devices are required to maintain the complementary bearing surfaces fully seated and rotation is permitted between the complementary bearing surfaces resulting in accelerated wear of the bearing surface of the base member.